The present application relates to the magnetic resonance imaging arts. It finds particular application in conjunction with magnetic resonance imaging of the torso of human subjects and will be described with particular reference thereto. It is to be appreciated, however, that the present invention also finds application in conjunction with imaging other parts of the human anatomy and with the imaging of non-human and inanimate subjects.
In magnetic resonance imaging, dipoles are selectively aligned with a primary magnetic field. Radio frequency excitation pulses are applied to stimulate resonance in the aligned dipoles and radio frequency magnetic resonance signals are collected from the resonating dipoles. Gradient magnetic field pulses are applied to encode spatial position. When imaging the upper torso, which includes the heart and other moving tissue, high speed image acquisition is advantageous.
To promote high speed image acquisition and high resolution, high strength magnetic field gradients with high slew rates are advantageous. That is, gradients of large magnitude that can be switched on and off very quickly improve data acquisition time and resolution. However, gradient strength varies inversely as the radius squared of the gradient coil and stored energy, a critical factor for slew rate, varies with the fifth power of the radius of the gradient coil.
To improve data acquisition speed at resolution in other parts of the human anatomy, smaller diameter gradient coils have been used, e.g., smaller diameter head or wrist coils. However, the width of the patient's shoulders has been a limiting consideration for upper torso imaging.
To improve the magnetic field gradient characteristics in the upper torso, elliptical gradient coils and planar gradient coils have been utilized.
Another drawback of whole body coils, insertable coils, and local coils is that they limit access to the examined patient. The gradient coils substantially surround the examined region. In order for a physician to gain access to the examined region, such as for a biopsy, the patient must be removed from the gradient coil assembly. Moving the patient relative to the gradient coil assembly also moves the patient relative to the resultant image. The moved patient needs to be reregistered with the diagnostic image.
Typically, a head coil is on the order of 30 cm in diameter; whereas, a whole body gradient magnetic field coil is about 65 cm in diameter. Larger diameter gradient coils are typically positioned close to the main magnetic field coils in order to minimize main magnetic field coil diameters. When the gradient and main magnetic field coils are placed closely adjacent, the gradient coil assemblies tend to induce eddy currents in the main magnetic field coil assembly. Shield gradient coils are typically disposed between the gradient and main magnetic field coils to inhibit eddy currents. However, the shield gradient coils increase gradient coil power consumption, typically by about a factor of 2.
The present invention provides a new and improved gradient magnetic coil assembly for magnetic resonance imaging which overcomes the above-referenced problems and others.